Anodized member and method for sealing anodic oxide coating

ABSTRACT

An anodized member includes a metal, and an anodic oxide coating that is formed on the surface of the metal. The surface of the anodic oxide coating includes a high-concentration layer that has a sealing metal content of 1.5 mmol/g or more. The high-concentration layer has a thickness of 0.15 μm or more.

Japanese Patent Application No. 2011-001374 filed on Jan. 6, 2011, is hereby incorporated by reference in its entirety

BACKGROUND

The present invention relates to an anodized member that exhibits excellent water resistance and corrosion resistance, and is obtained by forming an anodic oxide coating on aluminum, magnesium, titanium, or an alloy thereof, and particularly relates to a method for sealing an anodic oxide coating.

A porous anodic oxide coating is formed on a metal (e.g., aluminum, magnesium, or titanium) or an alloy thereof via anodization.

Such an anodic oxide coating includes a very thin barrier layer (thickness: several tens to several hundred nanometers) that is formed on the surface of a metal, and a honeycomb-like porous layer that is formed on the barrier layer and has a number of pores having a pore size of about 100 to about 600 angstroms.

When the metal is aluminum or an aluminum alloy, and it is desired to implement rust prevention, the thickness of the anodic oxide coating is normally set to 3 to 30 μm, and alumite sulfate prepared using a sulfuric acid aqueous solution is normally used as the electrolyte solution.

Oxalic acid or an organic acid may be used as the electrolyte solution depending on the application.

Such a porous anodic oxide coating is sealed in order to compensate for inferior corrosion resistance.

A porous anodic oxide coating is normally sealed by an Ni salt sealing treatment that immerses the porous anodic oxide coating in a nickel acetate aqueous solution, a hydration treatment using pressurized steam (water vapor), a two-stage sealing treatment that combines the Ni salt sealing treatment and the hydration treatment, or the like.

However, a porous anodic oxide coating subjected to such a sealing treatment may exhibit insufficient corrosion resistance. The applicants of this application have proposed a sealing method that pressurizes a sealing solution in a state in which the porous anodic oxide coating is immersed in the sealing solution, or further includes depressurization or electrolytic neutralization (see JP-A-2007-254784).

The sealing method disclosed in JP-A-2007-254784 is very effective for improving corrosion resistance (particularly alkali resistance). However, surface whitening may occur due to water (e.g., rain water) depending on the application of the aluminum product, so that the design of the aluminum product may deteriorate.

JP-A-2007-254784 states that a whitening prevention effect is obtained by immersing the product in a wealdy acidic aqueous solution (e.g., ammonium acetate aqueous solution) (hydration agent). The invention aims at achieving a further improvement over the related art.

SUMMARY

An object of the invention is to provide a method for sealing an anodic oxide coating that is effective for suppressing surface whitening due to water, and an anodized member that is obtained by the method and exhibits excellent water resistance.

According to one aspect of the invention, there is provided an anodized member comprising:

a metal; and

an anodic oxide coating that is formed on a surface of the metal,

a surface of the anodic oxide coating including a high-concentration layer that has a sealing metal content of 1.5 mmol/g or more, and

the high-concentration layer having a thickness of 0.15 μm or more.

According to another aspect of the invention, there is provided a method for sealing an anodic oxide coating comprising:

immersing a porous anodic oxide coating formed on a surface of a metal in a sealing solution that has a sealing metal ion concentration of 30 to 60 mmol/l and a fluorine ion concentration of 70 to 120 mmol/l; and

subjecting the porous anodic oxide coating to a steam sealing treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows anodic oxide coating sealing conditions and evaluation results.

FIG. 2A shows the cross-sectional SEM image of an anodic oxide coating obtained in Example 1 (sample No. 1), FIG. 2B shows the cross-sectional SEM image of an anodic oxide coating obtained in Example 2 (sample No. 2), FIG. 2C shows the cross-sectional SEM image of an anodic oxide coating obtained in Comparative Example 1 (sample No. 11), and FIG. 2D shows the cross-sectional SEM image of an anodic oxide coating obtained in Comparative Example 5 (sample No. 15).

DESCRIPTION OF EXEMPLARY EMBODIMENTS

An anodized member according to one embodiment of the invention includes an anodic oxide coating in which a high-concentration layer that has a sealing metal content of 1.5 mmol/g or more is present up to a depth of 0.15 μm or more from the surface of the anodic oxide coating.

The term “sealing metal” used herein refers to a metal that is deposited in the pores formed in the anodic oxide coating. The sealing metal is deposited by immersing the anodic oxide coating in an aqueous solution that includes Ni ions, Co ions, or Cu ions.

It was found that the sealing metal is distributed at a high concentration from the surface of the anodic oxide coating up to a given depth (thickness) to form a high-concentration layer, and the thickness of the high-concentration layer and the sealing metal concentration (content) therein significantly affect surface whitening.

For example, when subjecting the cross section of a sulfuric acid anodic oxide coating that has been sealed using an Ni salt sealing agent to energy dispersed spectroscopy (EDS) using a scanning electron microscope (SEM), a thin layer that contains Ni at a high concentration (i.e., high-concentration layer) is observed in the uppermost part of the anodic oxide coating, and the Ni content in the high-concentration layer is analyzed. It was found that excellent water resistance is obtained when the high-concentration layer has a depth (thickness) of 0.15 μm or more and a sealing metal content of 1.5 mmol/g or more (details thereof are described later).

A high-concentration layer having a depth (thickness) of 0.15 μm or more and a sealing metal content of 1.5 mmol/g or more is obtained by forming a porous anodic oxide coating on the surface of a metal, immersing the porous anodic oxide coating in a sealing solution that has a sealing metal ion concentration of 30 to 60 mmol/l and a fluorine ion concentration of 70 to 120 mmol/l, and subjecting the porous anodic oxide coating to a steam sealing treatment.

An anodic oxide coating that exhibits excellent corrosion resistance and water resistance may be obtained by combining the method according to one embodiment of the invention with the technology disclosed in JP-A-2007-254784. In this case, pressure may be decreased or increased in a state in which the porous anodic oxide coating is immersed in the sealing solution.

It is presumed that the anodized member according to one embodiment of the invention suppresses formation of boehmite due to water (e.g., rain water) since the surface (surface area) of the anodic oxide coating includes the high-concentration layer having a sealing metal content of 1.5 mmol/g or more and a depth (thickness) of 0.15 μm or more, so that an excellent whitening prevention effect is obtained.

The inventors conducted detailed studies of the anodic oxide coating sealing conditions and the results thereof as described below.

An aluminum alloy was immersed in a sulfuric acid aqueous solution, and an anodic oxide coating having a thickness of 8 to 13 μm was formed by direct-current electrolysis. The anodic oxide coating was washed with water, sealed, and subjected to SEM-EDS analysis and a water resistance test. The results are shown in FIG. 1.

The Ni ion concentration (metal ion concentration) and the fluorine ion concentration in the sealing solution were adjusted using a low-temperature sealing agent “L-100” (manufactured by Okuno Chemical Industries Co., Ltd.) and acidic ammonium fluoride.

The pH of the sealing solution was adjusted to 5.5 to 5.8 using aqueous ammonia.

In Example 1 (sample No. 1), the anodic oxide coating was immersed in the sealing solution having an Ni ion concentration of 36.6 mmol/l and a fluorine ion concentration of 78.9 mmol/l at 30° C. for 20 minutes, washed with water, and subjected to a steam sealing treatment at 155° C. for 20 minutes.

FIG. 2A shows the cross-sectional SEM image of the anodic oxide coating obtained in Example 1.

An Ni high-concentration layer was observed in the surface area of the anodic oxide coating. The thickness (depth) of the high-concentration layer was 0.36 μm, and the Ni (sealing metal) content in the high-concentration layer was 2.7 mmol/g (i.e., the mass ratio was analyzed by EDS, and the Ni content was calculated using the atomic weight (58.7) of Ni).

In FIG. 1, a case where surface whitening was not observed when the anodic oxide coating was immersed in purified water at 40° C. for 240 hours in the water resistance test was indicated by “Acceptable”, and a case where surface whitening was observed when the anodic oxide coating was immersed in purified water at 40° C. for 240 hours in the water resistance test was indicated by “Unacceptable”.

As shown in FIG. 1, the sample No. 1 exhibited excellent water resistance.

In Example 2 (sample No. 2), the anodic oxide coating was sealed in the same manner as in Example 1, except that the temperature of the sealing solution was set to 25° C. The thickness of the high-concentration layer obtained in Example 2 was 0.24 um, and the Ni content in the high-concentration layer was 1.8 mmol/g. The water resistance achieved in Example 2 was acceptable.

FIG. 2B shows the cross-sectional SEM image of the anodic oxide coating obtained in Example 2.

It was confirmed from the results of Examples 1 and 2 that it is preferable that the high-concentration layer have a sealing metal content of 1.8 mmol/g or more and a thickness of 0.24 μm or more.

In Examples 3 to 6 (samples No. 3 to No. 6), the Ni ion concentration and the fluorine ion concentration in the sealing solution were changed as shown in FIG. 1, and the water resistance test was performed in the same manner as in Example 1. The water resistance achieved in each Examples 3 to 6 was acceptable.

It was thus found that excellent water resistance is achieved when the sealing solution has a sealing metal ion concentration of 30 to 60 mmol/l and a fluorine ion concentration of 70 to 120 mmol/l.

Note that it is preferable that the sealing solution have a sealing metal ion concentration of 34 to 55 mmol/l and a fluorine ion concentration of 70 to 110 mmol/l.

In Comparative Example 1 (sample No. 11), the anodic oxide coating was sealed in the same manner as in Example 1, except that the fluorine ion concentration was set to 55.3 mmol/l (i.e., less than 70 mmol/l or less). In this case, the high-concentration layer had a thickness of 0.12 μm (i.e., less than 0.15 μm) and an Ni content of 1.4 mmol/g (i.e., less than 1.5 mmol/g). As a result, the water resistance achieved in Comparative Example 1 was unacceptable.

FIG. 2C shows the cross-sectional SEM image of the anodic oxide coating obtained in Comparative Example 1.

In Comparative Examples 2 to 4 (samples No. 12 to No. 14), the Ni ion concentration and the fluorine ion concentration in the sealing solution were changed as shown in FIG. 1. The water resistance achieved in Comparative Examples 2 to 4 was unacceptable.

In Comparative Example 5 (sample No. 15) (reference example), a high-temperature sealing treatment using nickel acetate was performed at 90° C. for 20 minutes.

In this case, the Ni high-concentration layer had a thickness of 0.14 μm and an Ni content of 0.4 mmol/g (see FIG. 2D).

As a reference, the low-temperature sealing treatment was performed in the same manner as in Example 1 without performing the steam sealing treatment (omitted in FIG. 1). In this case, the Ni high-concentration layer had a thickness of 0.14 μm, and the water resistance was inferior to some extent to that achieved in Example 1.

It was thus confirmed that it is preferable to perform the two-stage sealing treatment that performs the steam sealing treatment after the low-temperature sealing treatment.

It is presumed from the above experimental results that comparable results are obtained when using Co (atomic weight: 58.9) or Cu (atomic weight: 63.5). It is preferable to perform the low-temperature sealing treatment at 20 to 30° C. (solution temperature) for 15 to 30 minutes, and perform the steam sealing treatment at 140° C. or more for 20 to 60 minutes.

Although only some embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within scope of this invention.

For example, pressure may be decreased or increased in a state in which the porous anodic oxide coating is immersed in the sealing solution. In this case, the sealing solution in which the metal provided with the porous anodic oxide coating is immersed may be put in a pressure vessel, or a container that holds the sealing solution in which the metal is immersed may be put in a pressure vessel.

It is possible to promote a phenomenon in which the sealing agent included in the sealing solution is deposited in the pores by increasing the pressure inside the pressure vessel. When the pressure inside the pressure vessel is decreased, the sealing solution that remains in the pores is discharged (diffused). 

What is claimed is:
 1. An anodized member comprising: a metal; and an anodic oxide coating that is formed on a surface of the metal, a surface of the anodic oxide coating including a high-concentration layer that has a sealing metal content of 1.5 mmol/g or more, and the high-concentration layer having a thickness of 0.15 μm or more.
 2. The anodized member as defined in claim 1, the sealing metal being Ni, Co, or Cu.
 3. The anodized member as defined in claim 1, the high-concentration layer having a sealing metal content of 1.8 mmol/g or more and a thickness of 0.24 μm or more.
 4. The anodized member as defined in claim 1, the surface of the anodic oxide coating not whitening when immersed in purified water at 40° C. for 240 hours.
 5. A method for sealing an anodic oxide coating comprising: immersing a porous anodic oxide coating formed on a surface of a metal in a sealing solution that has a sealing metal ion concentration of 30 to 60 mmol/l and a fluorine ion concentration of 70 to 120 mmol/l; and subjecting the porous anodic oxide coating to a steam sealing treatment.
 6. The method as defined in claim 5, further comprising: decreasing or increasing pressure in a state in which the porous anodic oxide coating is immersed in the sealing solution.
 7. The method as defined in claim 5, the sealing metal ion being an Ni ion, a Co ion, or a Cu ion.
 8. The method as defined in claim 5, the sealing solution having a sealing metal ion concentration of 34 to 55 mmol/l and a fluorine ion concentration of 70 to 110 mmol/l. 